1. Field of the Invention
The present invention relates to olefin-based resin compositions, and more preferably halogen-free olefin-based resin compositions. These compositions are applied to coatings of electrical cables used in automobiles. The compositions preferably satisfy the requirements of automobile applications, such as wear resistance, flame resistance, tensile strength and flexibility. The invention also concerns electrical cables coated with such compositions.
2. Description of Background Information
Poly(vinylchloride) has been the coating material for automobile electrical cables. The reason is that this polymer has good mechanical strength, formability at extrusion with electrical cables, flexibility and paintability. The polymer also provides an inexpensive base material.
Recently, however, global environmental concerns have compelled the car industry to reconsider the choice of product types used for automobile parts, including the coatings of electrical cables. As a result, halogen-free resin materials are currently replacing poly(vinylchloride).
There have been investigations into wear-resistant resin compositions that do not generate toxic gases, such as halogen gases, when they are burned. Such compositions include halogen-free compositions containing a polyolefin-based polymer and a metal hydroxide as a flame retardant, as disclosed in Japanese patent applications published under Nos. HEI 7-176219 and HEI 7-78518, the disclosures of which are herein incorporated by reference in their entireties. Further, Japanese patent application published under No. HEI 7-182930, the disclosure of which is herein incorporated by reference in its entirety, describes a composition containing a polymeric material consisting of a polypropylene-type resin, a polyethylene treated with an unsaturated carboxylic acid and an ethylene-type copolymer, on the one hand, and a metal hydroxide, on the other.
However, when such compositions are used in order to retard combustion or to perform auto-extinction of the flame, metal hydroxides must be added to the compositions in large amounts. The mechanical properties of the compositions, such as wear resistance and tensile strength, are then deteriorated a great deal. In order to avoid such deterioration in mechanical strength, it has been contemplated to add polypropylenes or a high-density polyethylene which are relatively hard resins. However, the coated electrical cables then become less flexible and less formable.
An aspect of the present invention is therefore to provide a preferably halogen-free olefin-based resin composition which has well-balanced properties required for the coatings of electrical cable used in automobiles, such as wear resistance, flame resistance, tensile strength, flexibility, heat resistance and low-temperature resistance.
To this end, there is provided an olefin-based resin composition comprising:
(i) polymeric material in an amount of 100 parts by weight which includes:
(a) about 30 to 94 parts by weight of propylene polymer portion comprising at least one propylene polymer having a melt flow rate of up to about 5 g/10 min;
(b) about 1 to 20 parts by weight of at least one polyolefin, a proportion of about 0.1 to 10% by weight of which is structurally modified through maleic acid anhydride treatment, so that the at least one polyolefin has a Shore D hardness of at least about 50; and
(c) about 5 to 50 parts by weight of at least one olefin-based polymer, a proportion of about 0.1 to 10% by weight of which is structurally modified through maleic acid anhydride treatment, so that the at least one olefin-based polymer has a Shore A hardness of up to about 95; and
(ii) about 30 to 200 parts by weight of at least one metal hydroxide.
Preferably, the polyolefin portion treated with maleic acid anhydride (b) accounts for a proportion of about 5 to 20% by weight of the polymeric material (i), the olefin-based polymer portion (c) accounts for a proportion of about 5 to 30% by weight of the polymeric material (i), and the metal hydroxide product (ii) comprises at least one metal hydroxide in an amount of about 50 to 150 parts by weight relative to said polymeric material (i).
Preferably yet, the metal hydroxide product (ii) comprises at least one metal hydroxide in an amount of about 70 to 90 parts by weight relative to the polymeric material (i).
Suitably, the propylene polymer portion (a) comprises at least one of propylene-ethylene block copolymer in which propylene constitutes at least 50% by weight of the block copolymer, propylene-ethylene random copolymer in which propylene constitutes at least 50% by weight of the random copolymer, and propylene homopolymer.
Preferably, polyolefin portion treated with maleic acid anhydride (b) comprises at least polypropylene structurally modified through maleic acid anhydride treatment.
Typically, the olefin-based polymer portion (c) comprises at least ethylene-vinyl acetate copolymer structurally modified through maleic acid anhydride treatment.
The metal hydroxide product (ii) in the above composition preferably comprises magnesium hydroxide.
In a suitable embodiment, the metal hydroxide product (ii), e.g., magnesium hydroxide, is treated with silane coupling agent.
Preferably, such magnesium hydroxide includes a surface, at least part of which is treated with silane coupling agent.
Further, the silane coupling agent preferably comprises aminosilane coupling agent.
As understood from the foregoing, the olefin-based resin composition according to the invention is substantially free of halogen compounds.
The invention further concerns an electrical cable coated with such an olefin-based resin composition as mentioned above.